1. Field of the Invention
The present invention relates to an image forming process comprising steps of heating image forming particles containing a dye former arranged in accordance with a color signal, thereby heat-transferring the dye former onto an image receiving substrate, thereafter causing a color developing agent to adhere onto the heat-transferred dye former to provide color images.
2. Description of the Prior Art
Conventionally, in addition to one shot color image forming method described in, for example, U.S. Pat. No. 4,294,902, by which color images can be obtained only through one exposure stage and only one development stage, various proposals have been made as an image forming method in this field.
However, in the conventional image forming method, an image receiving substrate initially containing the color developing agent of dye former has been used. Thus, plain paper, which is a commonly available office supply, could not be employed as an image receiving substrate. Also, the dye former is colorless or light in color in its ordinary state. The dye former was vaporized through heating to react with the color developing agent to develop a color. After the color development, the dye former was not vaporized, while an acid material was provided as the color developing agent.
However, to heat-transfer the dye former, the heating at 120.degree. through 250.degree. C. was required. When an image receiving substrate containing the color developing agent of the conventional art was used, it was required to choose a color developing agent, which was not changed in quality or was not deteriorated even at the heating temperatures of the heat-transfer operation. Thus, the color developing agent was restricted.
Namely, on the assumption that a plain paper, which was impregnated with color developing agent such as tartanic acid, DL-mandelic acid, O-benzoylbenzoic acid or the like, was used as an image receiving substrate, the color developing agent was dissolved due to the heat of the heat transfer to cause the image forming particles to adhere to the image receiving substrate, thereby to decrease the color purity of the colored image. When the image receiving substrate was left for a longer period of time, the color developing agent was heat-transferred or evaporated, which made it difficult to be developed.
Moreover, according to the conventional image forming process, the dye former was permeated into the image receiving substrate in the heat-transfer process of the dye former. The dye former was heated until it reacted with the color developing agent, and was developed. Thus, more heat was required than was necessary for heat-transfer of the dye former.
Accordingly, for example, active clay was provided as a heat resisting color developing agent. Thus, so-called clay paper, wherein the active clay was applied upon the base paper, was used. Although the color developing mechanism of the dye former and the active clay is not clear, it was found through experiments that after the dye former has been brought into contact with the active clay, which contains moisture of approximately 2% or more by weight, the heating is performed for color development. Accordingly, to use the clay paper as an image receiving substrate, the quantity of heat for vaporizing the dye former from the image forming particles and the quantity of heat for color development are required. Thus, a heater with a larger quantity of heat was required to be used or the heating period was required to be rendered longer.
Referring to FIG. 1, when a dye former 4 is heat transferred from the image forming particles, onto the conventional image receiving substrate 3 having a color developing agent layer 2 containing the color developing agent 1, the dye former 4 permeates into the layer 2 to develop a color as in particles 5 to provide a colored image 6. In the conventional art, excessively vaporized dye former 4 remained on the image receiving substrate 3 as it was, on the colored image 6 as shown in FIG. 1. The excessive undeveloped dye former 4 was re-evaporated due to time passing or when it was left in a high temperature atmosphere. The dye former was spread to the other portions as in "a" in the drawing to cause fogging or decrease in the color purity. Also, the dye former was spread to the other image receiving substrate as shown in "b" of the drawing to cause pollution.